1. Field of the Invention
The present invention relates to a variable geometry system turbocharger equipped with a variable nozzle unit which can alter a passage area for (a flow rate of) an exhaust gas to be supplied to a turbine impeller side.
2. Description of the Related Art
In recent years, various developments have been made on a variable nozzle unit, which is disposed between a turbine scroll passage and a gas discharge port inside a turbine housing of a variable geometry system turbocharger in such a way as to surround a turbine impeller (see Japanese Patent Application Laid-Open Publication No. 2009-144545). A specific configuration of a conventional variable nozzle unit is as described below.
Inside a turbine housing, there is provided a shroud ring which covers outer edges of multiple turbine blades of a turbine impeller. Multiple first support holes are formed to penetrate the shroud ring at regular intervals in its circumferential direction. Moreover, a nozzle ring is provided at a position away from and opposed to the shroud ring in the axial direction of the turbine impeller, and is united to the shroud ring. The nozzle ring is provided with multiple second support holes formed to penetrate the nozzle ring at regular intervals in its circumferential direction in such a way as to match the multiple first support holes in the shroud ring.
Multiple variable nozzles are disposed circumferentially at regular intervals between a facing surface of the shroud ring and a facing surface of the nozzle ring. Each variable nozzle is rotatable in forward and reverse directions (opening and closing directions) about its shaft center which is parallel to the shaft center of the turbine impeller. Moreover, a first nozzle shaft is formed integrally with a side surface of each variable nozzle on one side in the axial direction. The first nozzle shaft of each variable nozzle is rotatably supported by the corresponding first support hole in the shroud ring. Furthermore, a second nozzle shaft is formed integrally with a side surface of each variable nozzle on the other side in the axial direction, and coaxially with its corresponding first nozzle shaft. The second nozzle shaft of each variable nozzle is rotatably supported by its corresponding second support hole in the nozzle ring. Here, when the multiple variable nozzles are synchronously rotated in the forward direction (the opening direction), a passage area for an exhaust gas to be supplied to the turbine impeller side increases. When the multiple variable nozzles are synchronously rotated in the reverse direction (the closing direction), the passage area for the exhaust gas decreases.
A surface side of the nozzle ring opposite from the aforementioned facing surface communicates with a turbine scroll passage. An annular seal cover is provided between an outer edge portion of the shroud ring and an inner wall surface of the turbine housing. The seal cover establishes airtight sealing (blocking) between a surface side of the shroud ring opposite from the aforementioned facing surface (an end surface side of the first nozzle shaft of each variable nozzle) and the turbine scroll passage. The surface side of the shroud ring opposite from its facing surface communicates with an outlet side of the turbine impeller.
In the state where the surface side of the nozzle ring opposite from its facing surface communicates with the turbine scroll passage, and where the airtight sealing (blocking) is established between the surface side of the shroud ring opposite from its facing surface and the turbine scroll passage by using the seal cover, the surface side of the shroud ring opposite from its facing surface communicates with the outlet side of the turbine impeller. Accordingly, while the variable geometry system turbocharger is in operation, a pressure acting on the end surface of the first nozzle shaft of each variable nozzle can be made substantially smaller than a pressure acting on an end surface of the second nozzle shaft thereof. Thus, each variable nozzle can be brought closer to the facing surface side of the shroud ring by way of the difference in pressure, so that a side clearance between the side surface of each variable nozzle on the one side in the axial direction and the facing surface of the shroud ring can be made as small as possible . As a consequence, it is possible to suppress a leakage flow from a gap between the side surface of each variable nozzle on the one side in the axial direction and the facing surface of the shroud ring, to stabilize a flow of the exhaust gas along a tip side portion (a portion from a mid-span side to a tip side) of each turbine blade, and to sufficiently enhance turbine efficiency of the variable geometry system turbocharger.